There has been known a syringe which can be filled with a medicinal liquid in a sterile manner (see, for example, Patent Document 1). The syringe disclosed in Patent Document 1 includes a syringe outer tube, a gasket contained in the syringe outer tube and slidable within the syringe outer tube, and a plunger connected to the gasket and operated to move the gasket, wherein a space surrounded by the syringe outer tube and the gasket is filled with a medicinal liquid. In the syringe according to Patent Document 1, the connection between the gasket and the plunger is accomplished through screw engagement. For this purpose, the gasket is formed with a female screw in an inner peripheral portion of the proximal end thereof, while the plunger is formed with a male screw on an outer peripheral portion of the distal end thereof. The screw threads of the female screw and the male screw are each uniform in height. In addition, the gasket has a plurality of projections which are projectingly formed on the outer peripheral portion of the gasket so as to extend along the circumferential direction and which are spaced from each other along the longitudinal direction of the gasket.
By filling the syringe configured as above with a medicinal liquid, a prefilled syringe can be produced. The prefilled syringe is produced in the following manner. First, an empty syringe outer tube having been sterilized is filled with the medicinal liquid under a sterile condition, and then the gasket is inserted into the syringe outer tube (first step). Next, the plunger rod is connected to the gasket, to complete the prefilled syringe (second step).
In the first step, the gasket is in a state in which each of the projections is pressed by the inner wall (inner peripheral surface) of the syringe outer tube. Therefore, at a portion of the female screw that corresponds to the projection (the portion which is located on the inner side of the gasket and in close proximity to the projection), the inside diameter is reduced, so that the height of the screw thread is increased. Upon transition to the second step under this condition in an attempt to connect the plunger rod to the gasket, engagement of the male screw of the plunger rod becomes difficult because of reduction in the inside diameter of the female screw of the gasket. Thus, there has been a problem that the connecting operation is not easy to carry out. Such a problem is generated particularly in the production of a prefilled syringe having a small-diameter syringe outer tube which has an inside diameter of 7 mm or less.
In order to solve this problem, it may be contemplated to reduce the height of the screw threads of the female screw in the gasket and the male screw on the plunger rod. In this case, however, a new problem is generated in that the force of screw engagement between the screw threads cannot be secured on such a level that the gasket will not be disengaged from the plunger rod when the plunger rod is operated.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2007-289677